Inertial navigation systems employ a triad of accelerometers in combination with a triad of angular rate sensors for providing velocity and position information to a navigational computer. Such systems provide continuous measures of vertical acceleration, (perpendicular to the earth's surface) and horizontal acceleration, (parallel to the earth's surface) and can determine vertical and horizontal velocity as well as vertical position and horizontal position by time integration of the vertical and horizontal acceleration.
Simple integration of vertical and horizontal acceleration to provide vertical and horizontal velocity, and integration of the velocities to provide vertical and horizontal position results in unbounded errors in the resultant vertical and horizontal axis information due to two causes. First, INS accelerometers measure the desired vehicle acceleration but in combination with the gravity field. Although the normal component of the gravity field can be accounted for, the anomalous component cannot. Consequently, this acceleration together with accelerometer and gyro instrument errors lead to navigation errors at Schuler and siderial frequencies which grow unbounded with time. Second, any vertical position error results in an error in the computer derived normal gravity component which causes the vertical position error in the INS to increase without bound.
A gravity gradiometer measures the gravity field independently of vehicle acceleration so it can be used to correct INS accelerometer outputs for the anomalous gravity field component. The unstable INS vertical channel is often remedied in practice by integrating a height sensor (depth gauge, altimeter, or surface ship sea level knowledge). The difference between inertial vertical position and that of the height sensor is used to stabilize the inherently unstable vertical channel.
The resulting navigation performance, although much improved over that of the conventional unaided INS, continues to exhibit growing errors at schuler and siderial frequencies due to INS instrument (including gravity gradiometer) errors.
If a gravity field map is available the integrated INS (conventional INS+gravity gradiometer sensor(s)+depth sensor) can implement a map matching mode. In this scheme mapped values of gravity field anomaly are compared with measured values in a filter to bound velocity and position errors.
Apart from the requirement for a gravity map, navigation performance is somewhat compromised because the core INS around which map matching navigation is implemented has unbounded velocity and position errors so in weak gravity signature regions navigation errors will tend to grow.
Although the prior art which integrates a conventional 3 axis INS with gravity gradiometer(s), and a height sensor greatly improves navigation performance in both the INS (no gravity map) and in the map matching mode, it does not fully utilize overall navigations system velocity error observability. If this velocity error observability is exploited these errors can be bounded even in the INS mode. The resulting improvement in the core INS mode, where now only east position error is unbounded, leads to improved map matching performance.